This invention relates to an asynchronous transfer mode (ATM) device and, in particular, to an ATM device which is included in an ATM network to provide a Constant Bit Rate (CBR) service as a network service.
In an ATM network of the type described, a data stream or string is transmitted through an ATM device in the form of ATM cells each of which has a fixed length.
Specifically, such an ATM cell consists of the fixed length of 53 bytes which is divisible into a payload field of 48 bytes and a header field of 5 bytes. Herein, the payload field is given user information or the like and may be therefore called an information field.
In the ATM network, a plurality of virtual channels (VCs) are prepared as communication channels to establish a connection between a user terminal and an ATM device and a connection between ATM devices. Furthermore, a cluster or group of the virtual channels (VCs) is referred to as a virtual path (VP). In this connection, a plurality of VCs can be included in a single virtual path (VP) or can be assigned to one transmission line.
Under the circumstances, the ATM device mentioned above is used to decide either only a VC or both a VC and a VP, through which the ATM cells are transmitted. To this end, a virtual path identifier (VPI) and a virtual channel identifier (VCI) are arranged in the header field of each ATM cell to identify both the virtual path and the virtual channel.
Specifically, such an ATM device has a plurality of input ports and a plurality of output ports and mainly carry out a switch operation to establish routes determined by such as a VC and a VP. Therefore, the ATM device is sometimes called an ATM switch board, an ATM switch, or an ATM exchange. When an ATM cell is given through each of the input ports (the input ATM cell is simply called an input cell hereinafter), the ATM device selects an output port through which the input cell should be sent, referring to the VPI and/or the VCI of the input cell. Once the output port is selected, the ATM device converts the VPI and/or the VCI into a VPI and/or a VCI that correspond to a next VP and/or a next VC. In addition, the switch operation is carried out to set up a route, and the ATM cell is sent as an output cell to the output port through the route.
Among the ATM networks, such an ATM device that only executes the switch operation concerned with the VP alone is especially called a VP handler while an ATM device that executes the switch operation concerned with the VC is called a VC handler.
Further explanation will be made about structure of the ATM device.
The ATM device includes a buffer that stores ATM cells, and a switch core that controls write/read operation of ATM cells into/from the buffer. In detail, the switch core has line interfaces that correspond to the input ports and the output ports, and a header translator that is connected to each line interface. Herein, one of the line interfaces coupled to the input ports, is called an input interface. On the other hands, another one of the line interfaces coupled to the output ports, is called an output interface.
Data transmission in the ATM network, which comprises the above mentioned ATM devices, is classified into three levels composed of a physical layer, an ATM layer, and an ATM adaptation layer (AAL). Specifically, the ATM layer mainly transmits the ATM cells and also multiplexes, divides, and routes the ATM cells. The AAL stands on the ATM layer and conceals the behaviour of the ATM layer from the upper layer.
Specifically, the AAL can provide the upper layer with four categorized services; a CBR service class, a variable bit rate (VBR) service class, an unspecified bit rate (UBR) service class, and an available bit rate (ABR) service class.
Among the above services, the CBR service is effective to transmit high isochronal traffic, such as voice signal or data through voice lines. This means that the traffic of the CBR service requires to maintain synchronicity between a transmitter and a receiver.
In order to indicate performance on transferring cells in the ATM network, a cell delay variation (CDV) can be stated as one of parameters. As mentioned before, an ATM cell that is output from a transmitter is transmitted to a receiver through the connection that is established in the ATM network. During the transmission, such an ATM cell suffers from various kinds of delays, such as a transmission delay on the transmission line and a switching delay in the ATM device, which is described later. Furthermore, the delay is also affected by an amount of the traffic on the transmission line through which each ATM cell is transmitted. As a result, each ATM cell might not always have the same delay, and therefore the delay time might vary in each ATM cell. In this case, a difference between a cell transfer delay and an average cell transfer delay on a connection, is called the CDV described before.
If the CDV fluctuates beyond its limitation, the synchronicity between the transmitter and the receiver can not be maintained in the system that provides the CBR service. As a result, the system users can not reproduce exactly the CBR service data such as the voice data. Thus, it is necessary to absorb as such effect of CDV as possible in the system that provides the CBR service.
One of ways to absorb the effect of the CDV, to regulate the traffic, and to maintain the transmission quality in the network, is a shaping method. The shaping method is to accumulate ATM cells in a memory for shaping, and to rearrange the ATM cells. Especially, such a shaping method is used in each output port side of the ATM devices and is called a port shaping method.
A conventional shaping method in the stage posterior to the output port (henceforth the conventional shaping method) is disclosed in Japanese Unexamined Patent Publication (JP-A) No. 8-163150.
As for the conventional shaping method, a shaping unit to shape ATM cells comprises a shaping FIFO, a monitor circuit to monitor the shaping FIFO, and a control circuit to control the shaping FIFO by responding to signals from the monitor circuit. The shaping unit is usually arranged in each stage posterior to the output ports of ATM devices. When the shaping unit receives ATM cells from an output port that the shaping FIFO corresponds to, the monitor circuit monitors a time when each of the ATM cells is accumulated in the shaping FIFO. In this end, each ATM cell is given each delay time, by accumulated in the shaping FIFO for a time monitored by the monitor circuit. Herein, each delay time is to adjust a cell transfer delay of each ATM cell to one predetermined value, that is, CDVs of the ATM cells are absorbed by given the delay times. When the monitor circuit recognizes that each of the ATM cells is accumulated in the shaping FIFO for each delay time, the monitor circuit outputs an output signal to control circuit at each ATM cell. The control circuit controls the shaping FIFO by receiving the signal from the monitor circuit. Thus, each ATM cell is given the above delay time by regulation of the control circuit and the monitor circuit, and the shaping FIFO sends out shaped ATM cells.
The conventional shaping method which uses the conventional shaping unit delays ATM cells for the delay times each corresponding to the ATM cells, outputs the accumulated ATM cells at a certain interval, and thereby shapes the ATM cells. Resides, since a maximum of the CDV that happens on the ATM network is guaranteed to be a given value on the network, the conventional shaping unit employs the maximum of the CDV as the maximum delay time of the accumulated ATM cells in the shaping FIFO.
As understood from the above, the conventional shaping unit can absorb differences between the CDVs of the ATM cells and sends out the output cells from the output interface at a certain interval by generating the delay time in the shaping FIFO.
However, the structure mentioned above is disadvantagous in that the hardware of the conventional ATM device becomes bulky since it is necessary to put the shaping units into each of the output ports of the ATM device.
Furthermore, it is necessary that additional units should be added as the shaping units to the conventional ATM device without utilizing hardware of the conventional ATM device.